• A unicorn ant with oversized mandibles is described from Cretaceous amber of Myanmar

• The exaggerated head structures composed a highly specialized trap for large prey

• Haidomyrmecine ants were probably solitary specialist predators

• Some basal lineages had a refined ecology shortly following the advent of ants

Summary

Ants comprise one lineage of the triumvirate of eusocial insects and experienced their early diversification within the Cretaceous. Their ecological success is generally attributed to their remarkable social behavior. Not all ants cooperate in social hunting, however, and some of the most effective predatory ants are solitary hunters with powerful trap jaws. Recent evolutionary studies predict that the early branching lineages of extant ants formed small colonies of ground-dwelling, solitary specialist predators, while some Cretaceous fossils suggest group recruitment and socially advanced behavior among stem-group ants. We describe a trap-jaw ant from 99 million-year-old Burmese amber with head structures that presumably functioned as a highly specialized trap for large-bodied prey. These are a cephalic horn resulting from an extreme modification of the clypeus hitherto unseen among living and extinct ants and scythe-like mandibles that extend high above the head, both demonstrating the presence of exaggerated morphogenesis early among stem-group ants. The new ant belongs to the Haidomyrmecini, possibly the earliest ant lineage, and together these trap-jaw ants suggest that at least some of the earliest Formicidae were solitary specialist predators. With their peculiar adaptations, haidomyrmecines had a refined ecology shortly following the advent of ants.

Life-like reconstruction of new Late Cretaceous Worker Ants Ceratomyrmex ellenbergeri

Systematic Paleontology

Ceratomyrmex New Genus

Diagnosis. Differs from all other extinct and living ants by the presence, in workers, of a long cephalic horn arising from between the antennal insertions and curved forward and by very long, scythe-like mandibles, their tips reaching above head near to the horn’s apex. The horn is spatulate apically, and its undersurface is densely setose, with a semi-circular brush of peglike spicules along edges. Two pairs of long trigger hairs are each flanking the apical portion of mandibles. The combination of distinct ocelli and subpetiolar process also serves to distinguish the genus from other haidomyrmecines.

Type Species.C. ellenbergeri, new species.

See Supplemental Information for description of type and additional material.

Etymology. Derived from Greek ‘‘keratos,’’ meaning ‘‘horned,’’ and ‘‘myrmex,’’ meaning ‘‘ant.’’ The specific epithet is a patronym for Sieghard Ellenberger who provided some of the fossils.